1. Field of the Invention
The present invention relates to a liquid crystal display device and, in particular, to a liquid crystal display device in which spacers having a uniform height are provided on a substrate.
2. Description of the Related Art
In the recent years, research has been conducted to promote practical application of a liquid crystal display device which not only has a high density and a high capacity but also is capable of providing high performance and displaying high-definition images. There are various types of such a liquid crystal display device available. Among others, an active matrix-type liquid crystal display device has been widely used for several reasons, such as that crosstalk between adjacent pixels is small, a high-contrast image can be obtained, images can be displayed in the transmissive mode, and the display area can easily be made larger. In this active matrix-type liquid crystal display device, a thin film transistor (TFT) as a switching element and a pixel electrode are provided on an array substrate at each of intersections between multiple scanning lines and multiple signal lines which are arranged so as to intersect with each other.
Liquid crystal display devices are commonly provided with spacers for obtaining a uniform gap between an array substrate and an opposite substrate facing the array substrate. There are spherical and pillar spacers. The spherical spacer is applied in a production process for pasting the array substrate and the opposite substrate together. The pillar spacer is formed integrally either on the array substrate or the opposite substrate by the photolithographic method in a production process of the substrate. Since the pillar spacer can be arranged in the vicinity of the intersections between the scanning lines and the signal lines without overlapping with pixel electrodes, it is possible to obtain a good display quality. As for the region where the pillar spacer is arranged, in order to maintain the uniformity of the cell gap, a region is selected so that both the array substrate and the opposite substrate can secure a stable flatness, and the spacers are arranged at the same position in each of the pixels (for example, see Japanese Patent No. 3680730 and Japanese Patent Application Publication No. 2004-245952).
In order to arrange the pillar spacers, regions are selected so that both the array substrate and the opposite substrate can secure a stable flatness. Then, the pillar spacers are respectively arranged at the same positions within the selected regions in the pixels.
The density of the spacers largely affects the basic characteristics of a liquid crystal display device. Not only is a certain level of density needed for maintaining the uniformity of the cell gap, but also the density of the spacers needs to be increased to achieve high durability against pressure applied to a glass surface. In a display device having a high spacer density, on the other hand, a hollow portion is generated inside of the display device when pressure is applied to the surface of the display device at a low temperature (hereafter referred to as low-temperature bubble) because the rate of contraction of liquid crystal is faster than that of elastic deformation of the spacers. Therefore, the display quality is significantly deteriorated. In order to solve this problem, a technology has been recently disclosed for increasing durability against local external pressurization while preventing generation of low-temperature bubbles by forming two kinds of spacers having different heights on a substrate (for example, see, Japanese Patent Application Publication No. 2005-189662).
In addition to the publication described above, various proposals have been made for an arrangement of two kinds of pillar spacers having different heights on a substrate (for example, see, Japanese Patent No. 3338025).
However, it is still necessary to increase the density of spacers in order to obtain high durability against pressure applied to the glass surface. A certain level of spacer density or above is also required to obtain a uniform cell gap. On the other hand, the spacer density has to be lowered in order to improve the device in terms of the performance against low-temperature bubbles. Therefore, there is a problem that it is extremely difficult to improve the device in terms of the performance against pressure applied to the glass surface and the performance against low-temperature bubbles at the same time.
Moreover, if a configuration is employed in which spacers having different heights are formed on a substrate in order to solve the problems described above, a new problem of increasing production costs arises due to an increased number of production processes.